This invention is directed to methods for reducing cavitation in impulse ink jet printing devices. The invention is further directed to ink compositions comprising antioxidant for use in impulse or drop on demand (DOD) ink jet printers. Additionally, the invention is directed to solvent-based, pigment and dye inks formulated for high resolution and low spread bar code and graphics printing applications on porous substrates such as Kraft paper or corrugated cardboard and other porous materials like web off-set paper. The inks of this invention are also for printing on non-porous substrates.
Ink jet printing is performed by discharging ink droplets from a print head to a substrate. The droplets are ejected through orifices or nozzles in the print head and are directed to the substrate to form an image thereon. In contrast to many other types of printing, there is no contact between the printer and the substrate in ink jet techniques.
Most of the ink jet printers known in the art may be characterized as either continuous, thermal, or impulse devices, depending upon the mechanism by which the ink droplets are directed to the substrate. In continuous ink jet systems, an essentially uninterrupted stream of ink is ejected from a nozzle and breaks up into droplets. The droplets bear an electric charge so that they can be deflected by an applied electric field which is modulated according to the particular image to be recorded. The electric field directs the droplets toward either the substrate or an ink re-circulating reservoir. The inks employed in conjunction with continuous ink jet systems typically comprise a colorant such as a dye or pigment, an electrolyte to facilitate droplet deflection, and a liquid vehicle to dissolve or disperse the colorant and the electrolyte. While the vehicle in many continuous-type inks comprises water, U.S. Pat. No. 4,142,905, in the name of Cooke, disclosed a water-free ink composition consisting essentially of a low molecular weight glycol, an inorganic salt electrolyte, and a dye which is soluble in the glycol.
With so-called xe2x80x9cimpulsexe2x80x9d or xe2x80x9cdrop-on-demandxe2x80x9d ink jet printers, image formation is controlled by changing the rate of energization of a piezoelectric transducer rather than by modulating in applied electric field. Ink is stored in the print head or nozzle until it is necessary to form an image on the substrate on demand. The printer is then activated to apply pressure to the ink and discharge a selected number of discrete ink droplets toward the substrate. These ink droplets need not bear an electric charge. Accordingly, impulse ink compositions can be free of corrosive substances such as water and electrolytes which continuous ink jet inks often comprise.
However, impulse ink jet printers present a number of problems which are not encountered in continuous ink jet systems. For example, unlike continuous ink jet printers, impulse printers typically are maintained in a stand-by or temporarily inoperative mode between printing cycles. Thus, the ink is allowed to stand and possibly solidify in the discharge orifices of the print head. Impulse printers normally begin a printing cycle with such material in place. Many of the start-up problems encountered with impulse printers are attributable to ink which has been allowed to stand in the discharge orifices during stand-by periods. Such material is less of a concern in continuous systems because there typically are fewer interruptions in the flow of ink. Even where ink is allowed to stand and solidify, it is more easily purged due to the considerably higher pressures at which continuous ink jet printers operate. Accordingly, impulse-type inks must be specially formulated to minimize start-up problems.
Numerous ink compositions for impulse ink jet printers are known in the art. However, many of these inks are not suitable for bar code printing applications on porous, non-porous, or fibrous substrates. As will be appreciated by those of skill in the art, an ink applied to a substrate such as paper will tend to migrate or wick along the fibers of the paper until the wicking forces are countered by the ink viscosity. The extent to which the ink wicks will be dependent upon both its viscosity and the porosity of the substrate. Where highly porous substrates such as Kraft paper or corrugated cardboard are employed, many inks tend to wick excessively, leading to blurry, ill-defined printed images. One approach to clearer, more well-defined print images has involved the employment of a rapidly evaporating ink composition. However, rapid evaporation of the impulse-type ink often leads to clogging of discharge orifices during stand-by periods. Moreover, such rapid evaporation compositions are less desired because they commonly contain volatile organic components (VOCs) that can be harmful to the environment and human health. Another approach to clearer, more well-defined print images has involved the use of a pigment as a colorant instead of a dye. Where pigments are used as the colorant, the particle size employed must be small enough to achieve reliable performance in the printhead. Water-based pigment dispersions are known in the art that satisfy the printhead performance requirement, however, the reliability of these dispersions in the present application are unknown. In addition, for water-based pigment dispersions, the current state of the art limits the driving frequency of the of the printhead to between 3 and 8 kHz, resulting in a slower printing operation and through-put. Moreover, water-based pigmented ink systems have two major drawbacks. First, they are unreliable as they tend to settle during storage and in ink reservoirs. Second, water-based pigmented ink systemsxe2x80x94especially when the particle size of the pigment gets smallerxe2x80x94tend to entrap more air which results in inconsistent jetting of ink drops.
In certain applications, it is necessary that the image created by an ink jet printer possess a relatively intense threshold color. For example, many optical character reading devices cannot read images unless they possess a minimum color intensity. Those skilled in the art will recognize that bar code images typically must possess a good print contrast signal (PCS) (preferably greater than about 90 percent) to be machine readable. However, many of the known techniques for increasing the color intensity of an inkxe2x80x94such as increasing the concentration of the colorantxe2x80x94often adversely affect important ink properties such as viscosity, surface tension, and stability.
Therefore, there exists a need for a solvent-based impulse-type ink jet ink composition capable of producing clear, well-defined, color-intense images on even porous and non-porous substrates.
The present invention provides a non-aqueous-based dispersion ink composition for use in impulse ink jet printers. Preferably, the ink compositions comprise from about 20 to about 50 percent by weight of a pigment dispersion. The pigment dispersion comprises from about 25 to about 45 percent by weight of a pigment, such as carbon black, from about 15 to about 45 percent by weight of a polymeric dispersant, and from about 25 to about 45 percent by weight of a dispersion medium. Preferably, the ink compositions further comprise from about 15 to about 40 percent by weight of a diol ether and from about 20 to about 60 percent by weight of a plasticizer.
The present invention also provides a method for preventing the formation of gas bubbles inside of a print head of an ink jet ink printer. The method is practiced by adding an antioxidant in the ink jet ink compositions. The antioxidant is used in an amount of up to about 2 weight percent of the ink jet ink composition.
The ink jet ink composition of the present invention improves the ability to store and use a pigment dispersion that is suspended uniformly without entraining a detrimental amount of air, particularly wherein the average particle size of the pigment is less than 1.0 micron.